The MSC (Mobile Switching Center) pool is a group of MSC nodes sharing in parallel the traffic generated from one and only one MSC pool area. BSC (Base Station Controller) or RNC (Radio Network Controller) radio access nodes belonging to an MSC pool area are connected to all the MSCs of the MSC pool.
When partial or full traffic load removal is needed in a MSC node, e.g. when to perform scheduled maintenance or when to perform load redistribution among the MSC pool members, the MSC Pool Redistribution function can be used. This minimizes traffic disturbances, e.g. during the maintenance work of the MSC.
The MSC Pool Redistribution function can also be used to quickly redistribute UE (user equipment) registration in order to balance the traffic load in the MSC pool when a new MSC or BSC/RNC node is added to or removed from the MSC pool.
MSC Pool Redistribution allows moving UE registrations from an MSC belonging to a pool to any other or to a specified target MSC pool member. The operator can specify the percentage of the UE registrations to be redistributed to any other or to a specific target MSC pool member.
If MSC Pool redistribution is ordered for a number of UE registrations, the MSC selects the UE registrations that are to be redistributed to other MSCs of the pool. The selection avoids selecting only subscribers that were more active than others.
FIG. 2 shows the prior art signaling sequence for redistributing a number of UE (100) registrations registered in MSC 1 (120) by showing the example of moving a single UE (100) registration to MSC 2 (124), where MSC 1 (120) and MSC 2 (124) are both members of the same pool of MSCs (130). The following steps are performed:    1) The operator initiates the procedure by issuing an O&M (operation and maintenance) instruction to redistribute UE 100 registrations. The MSC 1 120 selects the UE 100 registrations to be redistributed.    2) A UE 100 is performing a location update procedure, or initiates an originating call or a short message.    3) The BSC/RNC 110 identifies the handling MSC 1 120 using the NRI (network resource indicator) part of the TMSI (Temporary Mobile Subscriber Identity) contained in the received message.    4) The BSC/RNC 110 routes the message to the MSC 1 120.    5) MSC 1 120 checks if this UE 100 registration is marked for redistribution to another MSC 122, 124 of the pool of MSCs 130.    6) In case the UE 100 registration is marked for redistribution, then MSC 1 120 initiates TMSI reallocation using a Null-NRI in the new TMSI. The TMSI reallocation comprises also a non-broadcast LAI (Location Area Identity).            Each MSC in a pool 130 has assigned one unique non-broadcast LAI that it uses in case it wants to be offloaded. Each MSC in the pool 130 has to be aware of the non-broadcast LAI/RAI assigned to the other MSCs in the pool, because in case of redistribution, the target MSC will retrieve data (e.g. IMSI, security context, mobility context) from the offloaded MSC based on non-broadcast LAI.            7) Location Update Accept is send to the UE 100 from the MSC 1 120.    8) The UE 100 performs a new location update using the newly allocated TMSI containing the Null-NRI.    9) The BSC/RNC 110 does not recognize the Null-NRI and derives that a new MSC 122, 124 shall be selected from the pool of MSCs 130, using a selection logic for a new subscriber. The MSC 1 120, that is on redistribution, is blocked for new UE 100 registrations in the BSC/RNC distribution function. In this scenario, the BSC/RNC 110 selects MSC 3 124.    10) The BSC/RNC 110 sends the Location Update message to the MSC 3 124. MSC 3 124 registers the UE 100.    11) MSC 3 124 performs TMSI reallocation using its own NRI. Then MSC 3 124 contacts the HLR 150 (Home Location Register) and a Cancel Location procedure initiated from HLR 150 removes the UE 100 registration from the MSC 1 120 (not depicted in the figure).    12) MSC 2 120 sends Location Update message to the UE 100. Redistribution is finished and UE 100 registration is redistributed to MSC 3 124 of the pool.
However, UEs 100 can be attached to a MSC 120 not only via a BSC/RNC 110 radio access node, but also via a packet based radio interface. In this case the UE 100 is attached to the MSC 120 via a SGSN (Serving Gateway Support Node) or MME (Mobility Management Entity) packet node 140.
For the time being SGs (SGs-interface between a MME and a MSC) or Gs (Gs-interface between a SGSN and a MSC)-attached subscribers are not considered in the standard UE registration redistribution mechanism. In the prior art mechanism SGs/Gs-attached UEs cannot be selected, so these UEs cannot be redistributed to another MSC of the pool.
This implies that today it is impossible to completely offload a MSC, since the SGs/Gs-attached subscribers will remain registered in the MSC. With the strong trend towards LTE (Long Term Evolution) radio using the SGs attachment to the MSC, more and more subscribers will be attached to the MSC via SGs/Gs-interface. So this problem increases significantly, rendering the standard UE registration redistribution function useless.